In the drying of boards, particularly building boards, such as gypsum plaster boards or mineral-fiber boards, the boards transported through a drier are brought into contact with heated air.
The supply of drying air can be done through longitudinal ventilation, cross ventilation or cross ventilation with nozzles. In the case of longitudinal ventilation the drying air is supplied at one end of the drier, or, when the latter is subdivided into several zone at one end of a zone, and evacuated at the opposite end.
In the case of cross ventilation the air is supplied at several locations on the sides of the drier and evacuated at the opposite sides, whereby it is possible to achieve larger mass flows of drying air through the drier. The largest air mass flows can be guided in the case of cross ventilation via nozzles through the so-called nozzle drier.
In most cases, an air recirculation process is used, wherein a large part of the drying air is recirculated. This drying air, also called recirculated air, is heated outside the inner drier space. Only a small part of the drying air is discharged as outgoing air and a part corresponding to the outgoing air is supplied from outside as fresh air.
For warming the drying air, e.g. through burners, optionally through damper registers, fuel i.e. primary energy is needed, and for the supply of air by fans electric energy, i.e. secondary energy is needed. The primary energy as well as the secondary energy, estimated to be three times as costly, should be kept as low as possible.
In the DE-Z Zement-Kalk-Gips, No. 8 1991, Pages 421 to 425 a process for the drying gypsum wall-building boards with cross ventilation is described, wherein a lower consumption of primary energy is achieved by using the condensation heat of the outgoing air. For this purpose in each of the two drier zones the hot air is cooled down in a heat exchanger arranged between the drier pipes for preheating of the hot air. Since the hot air, i.e. the drying air supplied to the board, is heated only to low temperatures, a large mass flow of drying air is needed. This leads to a relatively high consumption of secondary energy.
A further drying process wherein a low consumption of primary energy is achieved by using the condensation heat of the outgoing air is known from DE-A 26 13 512. This process is a two-stage process. In the first drying stage high temperatures and high air humidity are used and in the second drying stage low temperatures and low humidity are used. The drying efficiency of the first stage is two or three times greater than that of the second stage and the second drying stage is heated by the outgoing air of the first drying stage, due to the interposition of a heat exchanger. In both stages the drying air is supplied in a recirculation process, namely in the first drying stage in the form of a longitudinal ventilation and in the second stage in the form of cross ventilation with a large mass flow of recirculated air. The large recirculated air mass flow of the second stage and the resulting high consumption of secondary energy are the reasons why in practice this process has been replaced by the process of the invention.
In the generic drying process known from the book xe2x80x9cTrocknungstechnikxe2x80x9d by K. Kroll and W. Kast, Third Volume, 1989, Pages 489 to 493, gypsum plaster boards which are guided through the drier on decks, are also dried in two stages at high temperature but at average humidity of the drying air in the first stage and with average temperature and low humidity of the drying air in the second stage. In the generic drier two zones are provided for performing the first stage and one zone is provided for performing the second stage. This and the higher temperatures of the drying air in the first stage lead to the assumption of a higher drying efficiency in the first stage. Due to the higher temperature of the drying air in the second stage when compared to DE-A 26 13 512, extremely large mass flows of recirculated air are avoided, so that this process leads to a low consumption of secondary energy. However the consumption of primary energy is relatively high.
In attempting to reduce the consumption of primary energy by using the condensation heat of the outgoing air, a general problem arises due to the fact that the waste heat of the outgoing air is available only at a low temperature level. Although a lower temperature of the drying air could be compensated by larger air mass flows, this would lead to a higher consumption of secondary energy, as described in the known process.
It is therefore the object of the invention to provide an improved process and drier for the drying of plasterboard and the like with the lowest possible consumption of primary and secondary energy. Specifically with the invention the used primary energy should be kept as low as possible by using the waste heat and also the condensation heat of the outgoing air, without increasing the requirement for secondary energy due to the recirculation of large air mass flows.
The process for drying boards which are guided through a drier on decks, according to the invention, contacts the boards with drying air in two stages A and B, whereby in stage A the drying is carried out in a recirculated-air process with high temperature and at least average air humidity and with a drying capability which is two to four times higher than in stage B, and in stage B the outgoing air of stage A can be guided through a heat exchanger arranged in the decks of the drier and the drying air at a low temperature and low humidity can be guided in counterflow to the outgoing air of stage A.
In stage A, drying air at a temperature of 150xc2x0 to 300xc2x0 C. and an air humidity of 0.2 to 0.8 can be supplied, drying air at a temperature of 120xc2x0 to 200xc2x0 C. and an air humidity of 0.2 to 0.8 is discharged, and a part thereof is directed into stage B, and in stage B drying air is supplied in the form of longitudinal ventilation, whereby the drying air with a temperature of 20xc2x0 to 80xc2x0 C. and an air humidity of 0.005 to 0.015 is supplied and drying air at a temperature of 80xc2x0 to 110xc2x0 C. and an air humidity of 0.03 to 0.1 is discharged.
The outgoing air leaving the heat exchanger in stage A B can be directed into a heat exchanger for preheating the drying air of stage B.
The drying air of stage A, guided as recirculated air, can be heated by at least one burner, the drying air discharged from stage B being directed towards the burners. Advantageously the boards are at first dried in a preliminary drying stage, are subsequently dried in stage A and are .finally dried in stage B.
Alternatively the boards are dried at first in stage B and subsequently in stage A.
A drier for drying boards, with a conveying device for transporting boards arranged on decks through the drier, can have a section A with at least one zone, which has a supply device, an evacuation device and a recirculated-air channel with conveying means and a heating device for recirculated air, as well as means for the supply of incoming air and means for the discharge of outgoing air, and a section B with a supply device for drying air and an evacuation device for drying air which are arranged at opposite ends of the drier.
A heat exchanger can extend in the decks above the conveying device through the section B of the drier, and can have a supply device and an evacuation device which are arranged at opposite ends of the drier, whereby the supply device of the heat exchanger is connected with at least one means for discharging the outgoing air of section A. The supply device of the heat exchanger and the supply device for the drying air of stage B can be arranged at opposite ends of the drier.
The drier can have a heat exchanger for preheating the drying air of section B, whose supply line is connected to the evacuation device of the first-mentioned heat exchanger. The heating installation of section A can consist of burners and the evacuation device for drying air of section B can be connected with the incoming air lines of a burner.
The conveying device can have roller conveyors or belt conveyors. The first-mentioned heat exchanger can have tubes running parallel to the travel direction. The tubes can be interrupted by collectors arranged transversely to the travel direction and to which they are connected.
In stage B the outgoing air of stage A with a high water vapor content, guided through the heat exchanger, is cooled down by the drying air with a lower temperature to the point that a part of the water vapor condensates.
The waste heat and condensation heat of the outgoing air of stage A reach the inside of the drier in the immediate environment of the boards to be dried and are transmitted to the boards in the form of radiation and convection heat. Heating devices arranged outside the drier are not required. Pipes for the recirculated air can be simplified or saved.
By using also the condensation heat, which is made possible by the low temperature of the drying air cooling the heat exchanger and the at least average air humidity of the outgoing air of stage A, the primary energy is intensively utilized.
Due to the fact that the drying air is guided in counterflow to the outgoing air of stage A guided through the heat exchanger, cooler drying air meets already cooled outgoing air. This insures the widest possible condensation of the water vapors contained in the outgoing air and improves the utilization of primary energy. The intensive utilization of the primary energy leads to considerable savings of primary energy.
Generally in stage B the drying is done at the most with one half of the drying power of stage A. Thereby a part of the heat is transferred to the boards through heat radiation, due to the arrangement of the heat exchanger in the decks of the drier. Therefore for the transfer of the second part of the heat through convection only a relatively small amount of drying heat is required. The secondary energy required for guiding this relatively low amount of drying heat is considerably lower in the process of the invention than the one needed in processes with similar low consumption of primary energy.
Therefore in the process of the invention the primary energy is utilized to the largest possible extent, without substantially increasing the need for secondary energy.
In stage A the drying can take place according to the recirculated-air process, whereby drying air with a temperature of 150xc2x0 to 300xc2x0 C. and an air humidity of 0.2 to 0.8 is supplied and drying air with a temperature of 120xc2x0 to 200xc2x0 C. and an air humidity of 0.2 to 0.8 is discharged. A part of this drying air is drawn from stage A as outgoing air and guided into the heat exchanger of stage B. In stage B the drying air is supplied through longitudinal ventilation, whereby drying air with a temperature of 20xc2x0 to 80xc2x0 C. and an air humidity of 0.005 to 0.015 is supplied and drying air at a temperature of 80xc2x0 to 110xc2x0 C. and an air humidity of 0.03 to 0.1 is discharged.
With the drying power of stage A which is two to four times higher and the foregoing process parameters, the primary energy is optimally used with the lowest possible consumption of secondary energy.
In comparison to the prior art process, the process of the invention can dispense with the burner used in the third zone of the prior art process.
The mass flow needed in the process of the invention is even smaller than in the stage B, i.e. in the third zone of the prior art process. The process parameters make possible such a reduced mass flow of the drying air in stage B, that the drying air can be supplied in the form of a simple longitudinal ventilation, i.e. the recirculation of the drying air and therefore the recirculated-air pipes can be fully dispensed with. The total amount of secondary energy needed in the process of the invention, taking into consideration the additionally required electric energy for guiding the outgoing air of stage A through the heat exchanger of stage B, is approximately as great as in the prior art process.
Since due to the heat exchanger surfaces the flow cross section in the inner space of the drier is also reduced, the smaller mass flow of drying air can be guided past the boards at a flow velocity which is almost equal to stage A.
The invention can utilize a preheating of the drying air supplied to stage B in a second heat exchanger outside the-drier, through which the outgoing air leaving the stage B is guided.
The heat contained in the drying air discharged from stage B is used by directing the drying air to the burners of stage A.
The process is particularly suited for drying gypsum plaster boards, which towards the end of the process are not supposed to be exposed to very high temperatures, because of the danger of gypsum calcination. Therefore after a preliminary drying, the boards are dried at first at high temperatures in the stage A and subsequently at lower temperatures in stage B. Besides this temperature course favors the starch migration which is needed for good cardboard bonding.
The alternative approach is particularly suited for drying boards, e.g. mineral-fiber boards, which could also be exposed in a dry state to higher temperatures. In this process the boards are predried in stage B and in stage A are dried to ultimate humidity. In this temperature course the drying takes place at high temperature differences of drying air and boards, which leads to a particularly efficient utilization of primary energy.
A roller-conveyor drier or a belt drier according to the invention can have several roller conveyors or belts arranged on top of each other, is particularly well suited for incorporating a heat exchanger which thereby extends in the decks above roller conveyors or belts.
The heat exchanger can have heat exchanger tubes running parallel to the conveying device or can have heat exchanger plates. The advantage of the tubes is a lesser danger of contamination of the heat exchanger, while the plates are easier to mount.
When the heat exchanger is equipped with tubes, these are advantageously interrupted by collectors arranged transversely to the conveying direction, whereby the tubes are connected to these collectors. In the collectors the condensate can be collected and discharged from there. In certain cases the collectors also facilitate the cleaning of the tubes, since through them cleaning devices can be introduced into the tubes.